Nonionic Micelles

(1) h n t addrees: Department of Biochemistry, Melbourne Uni- versity, Parkville ..... This rate constant is higher than that used in fitting the othe...
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Langmuir 1993,9, 117-120

117

8N2 Reactions of a Sulfonate Ester in Mixed Cationic/ Nonionic Micelles Clifford A. Bunton' and Sally W r i g h t 1 Department of Chemistry, University of California, Santa Barbara, California 93106

Paul M. Holland' General Research Corporation, P.O. Box 6770, Santa Barbara, California 93160

Faruk Nome Departamento de Quimica, Universidade Federal de Santa Catarina, 88049 Florianopolis, Santa Catarina, Brazil Received August 14,1992. In Final Form: October 27,1992

Addition of the nonionic surfactant CI& (~-CI$I~~(OCH~CH~)~OCH~CH~OH) to aqueous cetyltrimethylammonium bromide ((CTAIBr) inhibits the micellar-mediated reaction of B r with methyl naphthalene-2-sulfonate(MeONs) under conditions in which MeONs ie essentially fully micellar-bound. Fractional micellar ionization, a,of (CTAIBr is increased by CloE4,and the loss of Br from the micellar surface is a major cause of the inhibition. First-orderrate constantsat the micellar surfaceare proportional to the mole ratio of bound B r to total micellized surfactant, and second-order rate constants in the micellar pseudophase are very similar to those at the surface of a (CTAIBr micelle. These eecond-order rate constants are almost unaffected by incorporation of cl& or a moderately hydrophobic alcohol in the micellar pseudophase,probablybecausereaction occurspreferentiallyadjacentto cationichead groups at the micellar surface. The pseudophase model of micellar rate effects treata micelles and water as discrete reaction media so that the overall rate of reaction depends upon reactant concentrations and rate constants in aqueous and micellar pseudophases.2 The model has also been applied to reactions mediated by microemulsions,3reverse micelles,4 vesicles,5and hydrophobic ammoniumions.6 Partitioning of ions between the pseudophases has to be considered for reactions of nucleophiles, e.g. OH-, C1-, or Br-, in solutions of cationicmicelles. If the solution contains both reactive and inert counterions, their competition for the colloidal surface has to be considered,2" but this complication is avoided when a single type of counterion is present.2If a micellar surface is saturated with counterions,their concentration at the surface should be proportional to /3 = 1 - a (a is the fractional micellar ionization). For counterions that are not very hydrophilic a is approximately independent of the total ion concentration,2b+cand the rate of reaction in the micellar pseudophase should be unaffected by added reactive ions.' This prediction is satisfied reasonably well for reactions of dilute, weakly (1) h n t addrees: Department of Biochemistry, Melbourne University, Parkville, Victoria 3062, Australia. (2) (a) Mertinek, K.; Yataimiraki, A. K.; Levaehov, A. V.; Berezin, I. In Surfactant8 in Solution; Mittal, K. L., Ed.; Plenum Prese: New York, 1977;Vol.2, p 489. (b) Romted,L.S. In Surfactants in Solution; Mittal, K. L.,Ed.; Plenum Prees: New York, 1977; Vol. 2, p 509. (c) Romsted, L. S. In Surfactants in Solution;Mittal, K. L., Lindman, B., Eds.; Plenum P w : New York, 19&1;Vol. 2, p 1016. (d) Bunton, C. A.; Savelli, G. Adu. phy8. Or#. Chem. 1986, 22, 213. (e) Bunton, C. A. In Cationic Surfactants: Physical Chembtry; Rubingh, D. N.,Holland, P. M., Ede.; Surfactant Science Series 37; Marcel Dekker, Inc.: New York, 1990, p 323. (0Bunton, C. A.; Nome,F.;Quina,F.H.; Romsted,L.S. Acc. Chem. Rea. 1991,24,367. (3) Mackay, R. A Adu. Colloid Interface Sci. 1981, f6, 131. (4) (a) El Seoud, 0. A. Adu. Colloid Interface Sci: 1989, 30, 1. (b) OConnor, C. J.; Ramage, R. E.; Porter, A. Adu. Colloid Interface Sct. 1981, f6,26. (6)Kawamuro, M. K.; Chaimovich, H.; Abuin, E. B.; Lmi, E. A.; Cuccovia, I. M. J. Phys. Chem. lWl,%, 1468. (6) Bunton, C. A.; Hong, Y. S.;Romsted, L.S.; Quan, C. J. Am. Chem. sm. ~ .7 ~ . - - -.isai. - - - -, -1-0-,3- ..-_ (7) Bunton, C. A.; Gan, L.-H.; Moffatt, J. R.; Romsted, L.5.;Savelli, G. J. Phys. Chem. 1981,85,4118.

hydrophilic, anions, e.g., C1- or Br-, but even here ionic concentrationsat the micellar surface apparently increase modestly with totalion concentration.8 T h e rate increases can be fitted by equations of the Langmuir form? or by solutionof the nonlinear Poisson-Boltzmann quati0n.QJ0 Alternatively ionic concentrations in the micellar pseudophase can be estimated electrochemically,ll by fluorescence quenching,12 by NMR spectrometry,aJs or by trapping of phenyl cations.14 Some of these methods are not applicable at high ionic concentrations. Cationic micelles increase the rate of the sN2 reaction of Br- with methyl naphthalene-2-sulfonate (MeONs) in water, and rate-surfactant profiles can be fitted quantitatively in terms of the distribution of the two reactants between the micellar and aqueous pseudophases.8

MeONs

ONs-

Added 1-butanol slows the reaction in micelles of cetyltrimethylammonium bromide ((CTA)Br), largely because 1-butanol, like other organic solutes, decreases ionic concentrations at the micellar surface.16 "his decrease can be monitored conductimetridy, and the (8) (a) Bacaloglu, R.;Bunton, C. A,; Ortega, F. J. Phys. Chem.1989, 93,1497. (b) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; m a ,F. J. Phys. Chem. 1990,94,5068. (9) (a)Bunton, C. A,; Moffatt, J. R. J. Phys. Chem. ISM, 89,4168. (b) Bunton, C. A.; Moffatt, J. R.J. Phye. Chem. 1986,90,638. (c) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988,92,2896. (10) Ortega, F.; U e n a e , E. J. Phys. Chem. 1987,9f, 837. (11) (a) h a , R. J. Colloid Interface Sci. 1980, 78, 330. (b) Lianoa, P.; Zana, R. J. Colloid Interface Sci. 1982,88, 694. (12) Abuin,E.B.;Lmi,E.;Araujo,P.S.;Aleixo,RM.V.;Chaimovich, H.; Bianchi, N.; Miola, L.; Quina, F. J. Colloid Interface Sci. l W , 96, 293. (13) Bacaloglu, R.; Bunton, C. A,; Cerichelli, G.; Ortega, F. J. phy8. Chem. 1989,93,1490. (14) Chaudhuri, A.;Romted,L. S. J.Am. Chem.Soc., 1991,119,MS2. (16) Bertoncini, C. R. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94,6876.

0743-7463/93/2409-0117$04.00/0Q 1993 American Chemical Society

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second-orderrate constant for reaction of Br- with MeONs at the micellar surface is almost unaffected by l-butanol,16 although it is incorporated in the micelled4J6and could therefore affect their properties as reaction media. While the distribution of l-butanol between water and micelles has been measured,l4J8a mixed system is inherently simpler if we use a nonionic surfactant because at concentrationswell abovethe criticalmicelle concentration (cmc)nearly all of the surfactant will be micellized.16This eliminates uncertainties in the partitioning of the nonionic component between micellar and aqueous pseudophases. Because the presence of the nonionic surfactant may change micellar surface structure and thereby affect the rate constant in the micellar pseudophase, our study examines this question. The surfactant mixture was (CTA)Br (n-ClsHwNMe3We Br) and cl84 (n-CloH21(0CH2CH2)sOCH2CH2OH). used Br- as the nucleophile, rather than OH-which could deprotonate cl$&and generate a reactive alkoxide ion. The experiments were designed such that surfactant concentrations were always much higher than the cmc of either surfactant.

I\ I o\

I

k,C k , - kHZO (1) The spontaneous reaction of micellar-bound MeONs was not strongly affected by changes in the ratio of CTA(S04)1/2to Cl&. Fireborder rate constants, 10akH@,for the spontaneous reaction were 4.42, 4.39, 2.48, 1.85, and 1.54 s-l in 0.05 M CTA(SO~)IIZ with added 0, 0.01,0.04,0.06, and 0.09 M C&4, respectively. In 0.025 M CTA(QO4)1pvalues of 106kHBwere 3.10,2.33,1.62, and 0.69 s-l with added 0.005, 0.015, 0.025, and 0.05 M Cl&, respectively. Thew results indicate that reaction with C i a 4 is not of major importance.

Results Einetics. Addition of Cl& slows the reaction of Brwith MeONs in solutions of (CTA)Br, and values of kqC are shown in Figures 1and 2. We used various combinationsof surfactant concentrations,i.e., constant [(CTA)(16) Getth, J.; Hell, D.;Jobling, P.L.;Wing,J. E.;Wyn-Jones,E. J. Chem. Soc., Fara&y Thana. 2 1978, 74,1967. (17) Neves, M. de F.5.;Zanette, D.;Quina, F.;Moretti, M. T.;Nome, F.J. Phys. Chem. 1989,93,1502.

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[C,Cnlt M

Figure 1. Reaction of MeONs with Br- in (CTA)Br of concenwith added C i a 4 a t 25.0 OC. tration 0.025 M (0)and 0.05 M (0) The lines are theoretical.

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Experimental Section Materials. The preparation and purification of MeONs, (CTA)Br, and CTA(S04)1/2 have been described.& The C l a 4 was the single-species surfactant with a purity of 99.69% aa determined by gas chromatography. Conductance. Values of the fractional ionization, a,were determined from the ratio of slopes of ionic conductance above and below the cmc.11J7Measurements were made at fiied ratios of concentrations of (CTA)Br and Cl& at 25.0 OC on a YSI 35 conductance meter. This method cannot be used when cl&4 is in large e x c w over (CTA)Br becawe a is then 80 large that there is very little change of slope at the cmc. The breake in plots of conductivity against surfactant concentration were not as sharp as with aqueous (CTA)Br, and plots were h e a r below ca.0.4 mM and above 0.8 mM (CTA)Br. The conductance of 0.055 M (CTA)Brincrews on addition of Cd34 (04.045M) due to release of Br- from the micelles and a increases, although we cannot calculate these values because the composition of the micelle is changing. Khetics. The reaction of Br- with l W M MeONs in micellar solution at 25.0 "C was followed spectrophotometrically on a Beckman spectrometer at 326 nm. The spontaneous reaction with water was followed by using CTA(S04)1p with lo4 M H+ to suppress the reaction of OH-.% First-order rate constants, k,, 8-1, were correctedfor reaction with water (kH@) on the assumption that the counterion would not affect reaction of MeONs with HzO. There may also be a reaction with the hydroxyl group of Cl&, and we include any contribution in k H , p Corrected firstorder rate constants are given by

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Figure 2. Reaction at constant [CloE41 + [CTABrl = 0.05 M (Ohand at constant lCl&l + [(CTAIBrl = 0.05 M and total [Br-I = 0.05 M, with added NaBr ( 0 )or LiBr (D) at 25.0 "C. R = [(CTA)Brl/([(CTA)Brl+ [Cl&l). The lines are theoretical.

Brl and variable [CloE4I (Figure 11, and constant [surfactant] and variable [(CTA)Brl/[Cl&l without added Br- and with total [Br-I = 0.05 M (Figure 2). Here, and elsewhere, we write the terms in brackets as moles per liter of solution. Changes in the metal cation did not affect the reaction rate as shown in Figure 2, so possible interactionsbetween C& and the cation can be neglected. When reaction was followed with [(CTA)Brl+ [C&l = 0.05 M and the total concentration of Br- was kept at 0.05M by addition of MgBr2, the fmborder rate comtanta, 104kqC,s-l, were 6.78,5.07,3.30, and 1.59 for R = 0.9,0.7, 0.5,and 0.3,respectively (whereR = [(CTA)Brl/([(CTA)Brl + [ClJZJ)) and were within experimental error of values with added NaBr or LiBr (Figure 2). Fractional Micellar Ionization,a. Values of a were calculated from the ratio of slopes of plots of conductivity against [surfactant] at concentrations below and above the cmc, with constant ratios of [Cl&I/[(CTA)Br]. "'his method has been used e ~ t e n s i v e l y and ~ ~ Jgives ~ slightly higher values of a than those given by the Evans method which explicitly considersthe contribution of the micelles to the overall conductivity.&J* This treatment requires knowledge of the micellar aggregation number, which we do not have, and therefore we followed precedent in using the simpler method of slopes which gives eelf-coneietent values of a," We could not use thia method with [C&l/ (18) Evans, H. C. J. Chem. SOC. 1966, 679.

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Langmuir, Vol. 9, No. 1, 1983 119

s ~ Reactionu 2 of a Sulfonate Ester

Scheme I

s, +

Ks

DO

Table 11. Second-Order Bate Conmt.ntm in the Mimllu P#8UdoDhaM conditions 1@kM, 8-l 0.026 M (CTA)Br+ C I & ~ 1.00 0.06 M (CTA)Br + C1&4 1.16 [(CTA)Br]+ [C&l = 0.06 Mb 1.04 [(CTA)Brl+ [Cl&l= 0.06 Me 1.50 a Variable [surfactant]. b Constant [surfactant], variable [Brl. c Variable [surfactant], constant [Br-1 with added LiBr, NaBr, or MeBrz. ~

SM

pduds

Table I. Fractional Micellar Ionization, a,from Conductivity . [Cl&I/[(CTA)Brl R a [Cl&I/[(CTA)Brl R a 0.60 0.64 00 1.0 0.25 1.0 2.0 0.5 0.66 0.47 0.33 0.59 Reference 15.

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[(CTA)Br]> 2 because the difference in slopes above and below the cmc becomes too small as u 1. Plots of 8 = 1-a against R are linear, and we used these plots to calculate values of (3 by interpolation or extrapolation. The variation of u with R is shown in Table 1.

Diroussion Quantitative Treatment. T h e pseudophase model treats the overall reaction rate as the s u m of rates in the aqueous and micellar pseudophases (Scheme I). In this scheme subscripts W and M denote aqueous and micellar pseudophasee,respectively, S is substrate, D, is micellized surfactant (detergent), KSis the binding constant of S to micellized surfactant and kw' and kd are first-order rate constants for reaction of Br- with MeONs in the aqueous and micellar pseudophases,respectively. The overall rate depends on the distribution of the nonionic substrate between the micellarandaqueowpseudophases, e x p d by ita binding constant, Ks

where [D,] is the total concentration of micellized surfactant; Le., [D,]= [(CTA)Brl + [Cl&l in the present case with [surfactant] >> cmc. In single surfactant systems the concentration of micellized surfactant is generally taken to be the total concentration minus the critical micelle concentration, cmc.2 In mixed surfactant systems, this provides a useful "rule of thumb", but does not strictly hold due to a gradual increase in total monomer concentration above the cmc.19 In the present case, the cmca of (CTA)Br and C1& are 8 X lo-" Mm and 9.2 X lo-" M,2l respectively. Mixed cmca at the mixing ratios used in this study are lower than these values due to nonideal interactions in the mixed micelles, and are calculated to range from about 4.6 X 10-4 to 6.0 X 10-4 M on the basis of the strength of the nonideality effects for similar surfactant mixtures.21 Similar calculations of total monomer concentrations show these to remain below about 6 X lo4 M at the surfactant concentPations 0.026 and 0.06 M used in the experiments reported here. These results indicate that nearly all of the surfactant is in the m i c e k d form. The small correction for the reaction with water (Experimental Section) gives (3) The binding constant, Ks, is 1500 M-' with cetyltri(19) Holland, P.M.In Mixed Surfactant S y s t e m ; Holland, P. M., Rubingh, D. N., Eds.;ACS Symposium Series 501, American Chemical Society: Warhingbn, DC, 1992; p 114. (20) Mdcerjee, P.;MwIs, K.J. Criticol Micelle Concentrations of Aqwow Surfactant S y s t e m ; National Bureau of Standards! Washington, Dc,1970. (21)Holland, P. M.;Rubingh, D. N. J. Phys. Chem. l9%3,87,1984.

~

~~~~

methylammonium mesylate,'s so in our experimenta MeONs should be greater than 98% micellar bound. The situation is similar in the pmence of i-butanol.16 The second-orderrate constant for reaction of Br- with MeONs in water k, = 7.6 X lW6 M-l s-l at 26.0 OC,& so ',k can be neglected under our experimental conditions. The fmborder rate constant, k~',can be written as (4)

where k ~ s-',, is a second-order rate constant with the concentration of Br- in the micellar pseudophase written as a molar ratio of bound Br- to total micellized surfactant. The substrate is essentially fully bound under all our conditions; Le., Ks[D,I >> 1,and [Br-,] is given by 1- a = (3 = [Br-,l/[(CTA)Br] (5) Under our conditions surfactant concentrations are much larger than the cmc, and eqs 2-4 then simplify to

k,C = k,BR (6) where R is the mole ratio of cationic surfactant as defined earlier. Variations of k$C with surfactant concentration with no added Br- are fitted reasonably well by eq 6 with k~ r~ 1.0 X le3 s-l (Table I1 and Figures 1and 2). We ale0 fitted data for reaction in solutions of constant [Br-I (0.06 M) with k~ = 1.3 X s-l (Figure 2). This rate constant is higher than that used in fitting the other data, and the fit is worse. The problem is that we use values of a measured with no added NaBr (Table I) and addition of NaBr increases the concentration of Br- at the micellar surface,&*gb so our estimates of BrM or fl are too low and fitted values of k~ are consis*ntly too high. Changes in the salt cations (Na+,Li+,Mg+qdo not affect k E (Figure 21, although complexhg of cations with the poly(oxyethy1ene) residue of Cl&d should increase the charge of the micelle and its affinityfor B r . The absence of a rate effect suggests that these cationsinteract similarly with C&, or that this interaction does not increase the concentration of Br- at the surface of the mixed micelles. Some of our values of k~ (Table 11) differ slightly from preliminary values given in ref 22 because earlier we estimated u by interpolations of curved plots instead of linear plots of 1- u against R. Comparison with Other Surfactant Syrtemr. We compare values of k~ for reaction in mixed micelles of (CTA)Br and Cld& in water with values for reaction in aqueous (CTA)B+ and aqueous mixtures of it and l - b u t a n ~ l Valuesof .~~ l @ kare ~ 9.6 and 9.1 s-1for reactions in aqueous (CTA)Br and aqueous (CTA)Br + l-butanol, re~pectively.~J~ These values of kM are very similar to those used in fitting rate data for reactions in solutions of (CTA)Br + C& which range from 1X 10-8 to 1.3 X lO-9 8-1 (Table 11),although values of observed overall rata ~~

(22) Wright, S.; Bunton, C. A.; Holland, P. M. In Mixed SurJoctont Systems; Holland, P. M., Rubingb, D. N., Eds.; ACS Symponium Sorim 501, American Chemical Society: Washingon, DC, 1992; p 227.

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120 Langmuir, Vol. 9, No. 1,1993

constants vary over a wide range. It has been recognized that effects of micelles and similar assemblies upon rates of bimolecular ionic reactionsare govemed largelyby local concentrations of the two reactants at colloidal surfaces and for many reactions second-order rate constants are similar at the surfaces and in watere2 Second-orderrate constants at micellar surfacesshould depend upon the properties of the surfaces as reaction micromedia. In homogeneous solution second-orderrate constantsof reactions of anionicnucleophileswith nonionic substrates typically increase with decreasing polarity or water content of the solvent?3 although effects are large only for solvents of low water content. Polarities are lower at micellar surfaces than in bulk water, on the basis of spectral probes,aI25 and water activities are also lower.26 Polarities of oil in water (o/w) microemuleion droplets are similar to those at surfacesof ionic micelles, so addition of a hydrophobic alcohol, or a nonionicsurfactant, to a cationic micelle may not markedly decrease its surface polarity. In alcohol-modified micelles, e.g., of (CTA)Br + 1butanol, the alcohol is located at the micellar surface. Therefore,addition of alcoholto a given surfactant solution should generate more micellar surface, and dilution of reactants at this surface is accounted for in considering the distribution of reagents between water and micelles (Equation 2). We expected comicellization of c1&4and (CTA)Brto perturb the micellar surface so as to markedly affectvalues of, &M, if only because the four ethylene oxide residues are hydrophilic and too large to be accomodated at the micellar surfacewithout changing its structure. The similarity of values of k~ in (CTA)Brmicelles,&l-butanolmodified micelles,15and (CTA)Br-Cl& mixed micelles may be coincidental, although there are significant differences in the composition of these various assemblies. For example, the data in aqueous (CTA)Br are from experiments with and without added Br-, those with (CTA)Br + 1-butanol are with butanol in up to 30-fold excess over (CTA)Br15,and in the present work we used a range of concentrations of (CTA)Br, CloE4, and NaBr, LiBr, or MgBrz. Although added l-butanol and c1&4do not affect second-order rate constants, k ~at ,the micellar surface for reaction in aqueousmicelles, they are increased by an increase in the bulk of the cationic head group, by a factor of ca. 2.6 in going from the Me3N+ to the Bu3N+ head group, i.e., by decreasing micellar surface polarity.8 Moat quantitative treatments of micellar rate effects upon bimolecular reactions involve the assumption that the micellar pseudophase can be regarded, for kinetic purpw: as a uniform readion region? However, there is extensive contact between water and the alkyl tails,and the surface has ionic head groups surrounded by apolar regions.n We can rationalize our data for the mixed systems, and the variable head group sizes, by assuming that the ionicreagent, e.g., Br-, locates preferentially close to the ammonium head group, and that it reacts with substrate in this region, but that a nonionic substrate, e.g., MeONs, is diitributed uniformly over the micellar surface which may include l-butanol or C1&4 as well as (CTA)Br. In this hypothesis reaction occurs in a region whole properties are govemed by the trialkylammonium head group rather than by added nonionic solutea or (23) Lowry,T.H.;Richanleon,K.S.Mechaniemand TheoryinOrganic Chemistry, 3rd ed.; Harper and Row: New York, 1987; p 361. (24) Cordes, E. H.; Gitler, C. h o g . Bioorg. Chem. 1975,2,1. (26) (a) Z a c h a r i i , K. A.; Phuc, N. V.; Kozankiewin, B. J. Phys. Chem. 1981,86,2676. (b)Ramachandran, C.; Pyter, R. A.; Mukerjee,P. J. Phys. Chem. 1982,86,3198. (26) Angeli, A. D.; Cipiciani, A.; Germani,R.; Savelli, G.;Cerichelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1988, 121, 42. (27) Gruen, D. W. R. Prog. Colloid Polym. Sci. 1985, 70, 6.

surfactants. In this event k~ should not be very sensitive to these added solutes, even though they are incorporated in the micelle. However, these added solutes will affect theoverall rateof reaction,because they affectthetransfer equilibria of reactants between water and micelles, but these transfer equilibria are accounted for quantitatively by the pseudophase model, for example, by Scheme I and the related equations (eqs 3-61. Second-orderrate constante,k ~s-l,, cannot be compared directly with second-order rate constants in water with the dimensionsM-l s-l. However, concentration as amole ratio at the micellar surface can be written as a molarity in terms of the molar volume, VM,of the reactive region at the surface. Values assigned to this volume for a (CTA)X micelle in water range from 0.14 to 0.37M-' and may depend upon the nature of the reaction, and addition of a hydrophobic alcohol or C I ~ E I~f VM . ~ = 0.14 M-l, as assumed for (CTA)+micelles in water?& the second-order rate constant, k p , M-ls-l kZm = k,VM (7) is ca. 1.3 X lo4 M-l s-l, for aqueous (CTA)Br, which is approximately twice the value for the reaction in water.8b If we assume that VM has the same value in (CTA)Br micelles and mixed micelles with c&, our conclusions regarding the similarity of values of AM will also apply to values of kzm, where concentration is written as molarity at the micellar surface. Our treatment is simple and is applicable only in the limiting conditions in which substrate is extensively micellar-boL ' and a (or 8) can be measured directly. It fits the extensive body of evidence that the concentration of reagents at surfaces of colloidal assemblies is a major source of rate enhancements in many ion-molecule reactions? although for a few reactions second-order rate constants in aqueous and micellar pseudophases are so different that this generalization does not apply.*f#28

Acknowledgment. Support by the National Science Foundation (Organic Chemical Dynamics and htemational Programs) is gratefullyacknowledged. The authors are grateful to Dr. Robert G. Laughlin of The Procter & Gamble Co. for supplying UB with the Cl& used in this Symbols micellized surfactant binding constant, M-l, of substrate based on concentration of micellized surfactant first-order rate conatant, 8-1, with respect to substrate in the aqueous pseudophase firet-order rate conatant, s-l, with respect to substrate in the micellar pseudophase corrected overallfirst-orderrate constant,k,p = k+

+ kHz0

second-orderrate constant, M-1s-1, in the aqueous pseudophase second-orderrate conatant, M-18-1, in the micellar pseudophase second-order rate constant, 8-1, in the micellar pseudophase ratio of (CTA)Br to total surfactant substrate in the aqueous pseudophase substrate in the micellar pseudophase degree of fractional micellar ionization fractional counterion binding (neutralization)of micelle, p = 1- a ~~~

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~

~~~

~~~

(28) (a) Correia, V. R.; Cuccovia, I. M.; Chaimovich, H. J. Phys. Or#. Chem.1991,4,13. (b)Bacaloplu,R.;BlaskhA.;Bunton,C.A.;Foroudian, H. J. Phys. Org. Chem. 1992,5, 171.